For a long time, it was just setting the default search provider to Google in exchange for a beefy stipend. Later, paid links in your new tab page were added. Then, a proprietary service, Pocket, was bundled into the browser - not as an addon, but a hardcoded feature. In the past few days, we’ve discovered an advertisement in the form of browser extension was sideloaded into user browsers. Whoever is leading these decisions at Mozilla needs to be stopped.

Mozilla garnered a lot of fully deserved goodwill with the most recent Firefox release, and here they are, jeopardising all that hard work. People expect this kind of nonsense from Google, Apple, or Microsoft - not Mozilla. Is it unfair to judge Mozilla much more harshly than those others? Perhaps, but that's a consequence of appealing to more demanding users when it comes to privacy and open source.

I wouldn't quite say they abandoned Thunderbird. Their argument that it's functionally completed software (other than bug fixes and simple maintenance) is technically true for the time being, given that it is an e-mail client, and it does everything an e-mail client needs to do, in many cases more correctly or sanely than many competitors.

In order to manage these two perspectives, we are proposing to adapt the Thunderbird release and governance model in a way that allows both ongoing security and stability maintenance as well as community driven innovations for the product. We are opening this plan for discussion to individuals and organizations interested in maintaining and advancing Thunderbird in the future. We are looking for your feedback, comments and suggestions to refine and adapt the plan in the best possible way.

1. Firefox and Thunderbird have lived with competing demands for some
time now. Today Thunderbird developers spend much of their time
responding to changes made in core Mozilla systems and technologies. At
the same time, build, Firefox, and platform engineers continue to pay a
tax to support Thunderbird.

2. These competing demands are not good for either project. Engineers
working on Thunderbird must focus on keeping up and adapting Firefox’s
web-driven changes. Engineers working on Firefox and related projects
end up considering the competing demands of Thunderbird, and/or
wondering if and how much they should assist Thunderbird. Neither
project can focus wholeheartedly on what is best for it.

3. These competing demands will not get better soon. Instead, they are
very likely to get worse. Firefox and related projects are now speeding
up the rate of change, modernizing our development process and our
infrastructure. Indeed, this is required for Mozilla to have significant
impact in the current computing environment.

4. There is a belief among some that living with these competing demands
is good for the Mozilla project as a whole, because it gives us an
additional focus, assists Thunderbird as a dedicated open source
community, and also supports an open source standards based email
client. This sentiment is appealing, and I share it to some extent.
There is also a sense that caring for fellow open source developers is
good, which I also share. However, point 2 above — “Neither project can
focus wholeheartedly on what is best for it” -- is the most important
point. Having Thunderbird has an additional product and focus is *not*
good overall if it causes all of our products — Firefox, other
web-driven products and Thunderbird — to fall short of what we can
accomplish.

5. Many inside of Mozilla, including an overwhelming majority of our
leadership, feel the need to be laser-focused on activities like Firefox
that can have an industry-wide impact. With all due respect to
Thunderbird and the Thunderbird community, we have been clear for years
that we do not view Thunderbird as having this sort of potential.

6. Given this, it’s clear to me that sooner or later paying a tax to
support Thunderbird will not make sense as a policy for Mozilla...

I do wish they had released an android client before terminating the project. I still use thunderbird as my primary email & calendar software, but there are several longstanding bugs that have remained open and nobody's fixing them. I'm affected by an SSL bug and a webdav password manager bug, but the bugtracker just recommends server side workarounds as nobody's fixing the client.

Open source software is often trumpeted as living forever because anyone can fork the code and continue development. But it makes me wonder: as years turn into decades, how many (popular) open source packages are going to fall due to developer abandonment.